This study will capitalize on the opportunity for a natural experiment in HIV prevention to assess the implementation of a new syringe access law, Senate Bill 41 (SB41), in the inland counties of California's Central Valley. Injection drug use rates in the Central Valley's southern counties are among the highest in the U.S. and contribute substantially to the region's HIV epidemic. SB41, which went into effect on January 1, 2012, allows pharmacies to sell up to 30 syringes without a prescription. Although the law has the potential to greatly expand syringe access in inland California's predominantly rural and Latino counties, which have thus far been greatly underserved by other syringe access programs, syringe sales under the law are voluntary and it is unclear to what extent pharmacies will participate. Our study will use an innovative mixed methods approach to (1) assess pharmacy implementation of this new over-the counter (OTC) syringe law, (2) characterize utilization of OTC syringe sales by injection drug users (IDUs) under the law, and 3) examine structural factors associated with both pharmacy implementation and IDU utilization. The study will be conducted in two counties, Fresno and Kern, using a three-phase approach to address factors at all three levels of the HIV risk environment. In Phase I, we will conduct a syringe purchase trial (N=298), a brief pharmacy survey (N=596), and key informant interviews (N=32) to assess pharmacies' implementation of the law and characterize related barriers and facilitators. In Phase II, we will conduct an IDU survey (N=550) to understand IDU uptake of OTC syringe purchase under the law and identify related barriers and facilitators. In Phase III, we will conduct geospatial/contextual modeling to complement findings from Phases I & II and elucidate structural factors that influence SB41's impact. At the conclusion of our study, we will have generated a comprehensive understanding of the factors that facilitate and impede SB41's success as an HIV prevention tool. This information will assist the communities of inland California in developing multi-level strategies to optimize OTC syringe sales, reduce syringe sharing, and prevent HIV transmission among IDUs.